1. Field of the Invention
The present invention relates to the grounding and RFI isolation of control stations in general and more particularly to such control stations which have the need for remote location ground variation compensation and improved RFI isolation.
2. Description of the Prior Art
Control stations are known to have remote signal inputs thereto which may originate up to a mile from the station. This geographical difference causes ground potential differences which need to be considered. Also, in certain worldwide markets RFI isolation (containment of the RFI field within the cabinet) requirements are being made more stringent. This is especially true in the European market.
To date, prior art techniques for ground isolation did not consider such geographical ground potential differences due to remote signal origins and depended on mere metal shielding for RFI isolation.
Distributed process control systems such as the ELSAG BAILEY SYMPHONY.TM. system which utilizes control stations referred to as Operator Interface Stations or consoles have a major problem in complying with the requirements for obtaining a CE Mark for compliance with European RFI emission. This is due to the lack of sufficient high frequency grounding for the consoles front and rear door. The consoles are designed to have a 12 gauge green/yellow wire that is several inches long, connecting the door to the chassis of the console. The wire was added as a safety connection to meet UL (Underwriter Laboratories) and CSA (Canadian Standards Association) requirements. These requirements were for metal access doors to be grounded to the chassis so that only "hot" wires touching the door will blow a breaker/fuse and not produce a harmful voltage on the chassis which could cause the operator or service person to come into contact with this voltage. While the wire works for a safety ground it is insufficient for higher frequencies isolation.
The following rules apply to conductors carrying high frequencies:
a. Increasing rectangular conductor thickness by 1 .crclbar.x decreases induction by 10%. PA1 b. Increasing the width of the induction by 1 .crclbar.x decreases induction by 30%. PA1 c. decreasing the length of a rectangular conductor by 1 .crclbar.x decreases induction by 93%. PA1 a. The ground connections must consist of a wide and short electrical connection to the chassis. This is to provide proper RF grounding of the door. PA1 b. In addition, a more positive connection of door to chassis was needed to replace the green/yellow wire to account for a safety ground not easily disconnected by customers. PA1 c. The grounding scheme should be an integral part of the door and chassis and should be simple to use requiring no disconnection to access the internal electronics.
Thus, a long wire makes an extremely poor ground as the inductance becomes extreme even at only a few inches. The best conductor for RF frequencies is an extremely short `wire`.
Another problem with ground wiring is that the system is easily bypassed by some customers. Once the door is opened, the screw which holds the wire connected to the chassis must by removed to gain access to the console electronics therein. Often the wire is left unconnected when the door is replaced.
Thus the door grounding was needed which would have the following requirements:
Thus with the advent of new process control equipment which receives remote signal inputs from up to a mile away and which control equipment may be located in environments of high RFI sourcing, an improved control station which compensates for such geographical ground potentials and also has a very effective RFI isolation cabinetry was required.